Most would know that diesel engines, even with the same cubic capacity as that of a comparable petrol engine, produce significantly higher torque figures than petrol engines. This article aims to give a simple explanation on why this is the case.
Torque is a function of the force developed atop the piston (due to combustion) and the length through which it acts (the length of the connecting rod). If either of these two increases, torque increases. Let us assume here that the force produced due to combustion is the same for both the same-displacement petrol and diesel engines. Then, the differentiating factor is the length of the connecting rod.
By virtue of their design, diesel engines have higher compression ratios than petrol engines. Compression ratio increases if more parts of charge gets compressed to one part. That is, instead of 6 parts of charge getting compressed to 1 part, if 10 parts get compressed to 1 part, we say that the compression ratio is higher. To compress more parts of charge, the intake of charge should be higher and more specifically the length of the charge mixture needs to higher. Fluids take up the shape of the container that they fill up. So, in diesel engines, to have compress more parts of charge, the stroke of the engine is made to be larger than the bore. This configuration is referred to as the 'under-square' configuration.
Larger stroke means larger length through which the force acts. That is why diesels are inherently more torquier than their petrol counterparts. This is also the reason why they rev lower than petrol engines.
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